[en] The complex interplay between turbulence, magnetic fields, and self-gravity leads to the formation of molecular clouds out of the diffuse interstellar medium (ISM). One avenue of studying this interplay is by analyzing statistical features derived from observations, where the interpretation of these features is greatly facilitated by comparisons with numerical simulations. Here we focus on the statistical anisotropy present in synthetic maps of velocity centroid data, which we derive from three-dimensional magnetohydrodynamic simulations of a turbulent, magnetized, self-gravitating patch of ISM. We study how the orientation and magnitude of the velocity anisotropy correlate with the magnetic field and with the structures generated by gravitational collapse. Motivated by recent observational constraints, our simulations focus on the supersonic (sonic Mach number $\mathcal{M}\approx <!--\; ???\; -->2\text{--}17$) but sub- to trans-alfvénic (alfvénic Mach number ${\mathcal{M}}_{\mathrm{A}}\approx <!--\; ???\; -->0.2\text{--}1.2$) turbulence regime, and we consider clouds that are barely to mildly magnetically supercritical (mass-to-flux ratio equal to once or twice the critical value). Additionally we explore the impact of the turbulence driving mechanism (solenoidal or compressive) on the velocity anisotropy. While we confirm previous findings that the velocity anisotropy generally aligns well with the plane-of-sky magnetic field, our inclusion of the effects of self-gravity reveals that in regions of higher column density, the velocity anisotropy may be destroyed or even reoriented to align with the gravitationally formed structures. We provide evidence that this effect is not necessarily due to the increase of ${\mathcal{M}}_{\mathrm{A}}$ inside the high-density regions.

[en] The p and s-polarized surface plasmon polaritons (SPPs) of symmetric and asymmetric slabs formed arbitrarily by four types of conventional materials: dielectrics, negative dielectric permittivity materials, negative magnetic permeability materials, and left-handed materials are comprehensively analysed. The existence regions, dispersion relations, and excitation of SPPs in different frequency regions are investigated in detail. For symmetric slabs, the numbers and the frequency positions of surface polariton branches are quite different. At the same time, the pairs of the p or s-polarized SPP branches occur in the same frequency range. For asymmetric slabs, the SPP branches in mid- and high-frequency ranges are greatly different. In addition, the slab thickness has a great effect on SPPs of asymmetric and symmetric slabs. The attenuated total reflection spectra for the cases of p and s polarizations in these slabs are also calculated. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

[en] The Zeeman effect has been the only method to directly probe the magnetic field strength in molecular clouds. The Bayesian analysis of Zeeman measurements carried out by Crutcher et al. is the only reference for cloud magnetic field strength. Here we extended their model and Bayesian analysis of the relation between field strength (B) and volume density (n) in the following three directions based on the recent observational and theoretical development. First, we take R, the observational uncertainty of n, as a parameter to be estimated from data. Second, the restriction of α, the index of the B–n relationship, is relieved from [0, 0.75] to [0, 1]. Third, we allow f, the minimum-to-maximum B ratio, to vary with n. Our results show that taking R as a parameter provides a better fitting to the B–n relationship and much more reliable estimates on R, f, and the changing point of α. Arguably our most important finding is that α cannot be reliably estimated by any of the models studied here, either from us or Crutcher et al., if R > 2, which is indeed the case from our estimate. This is the so-called errors-in-variables bias, a well known problem for statisticians.

[en] Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the Submillimeter Array (SMA) and the Caltech Submillimeter Observatory (CSO) covering parsec-scale clouds to milliparsec-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment δ—the local angle between magnetic field and dust emission gradient—is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles (|δ|) fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal (|δ|) distributions are found for the separate SMA and CSO samples. Combining both samples broadens the distribution with a wide maximum peak at small (|δ|) values. Assuming the 50 sources to be representative, the prevailing source-magnetic field configuration is one that statistically prefers small magnetic field misalignments |δ|. When interpreting |δ| together with a magnetohydrodynamics force equation, as developed in the framework of the polarization-intensity gradient method, a sample-based log-linear scaling fits the magnetic field tension-to-gravity force ratio (Σ _B) versus (|δ|) with (Σ _B) = 0.116 · exp (0.047 · (|δ|)) ± 0.20 (mean error), providing a way to estimate the relative importance of the magnetic field, only based on measurable field misalignments |δ|. The force ratio Σ _B discriminates systems that are collapsible on average ((Σ _B) < 1) from other molecular clouds where the magnetic field still provides enough resistance against gravitational collapse ((Σ _B) > 1). The sample-wide trend shows a transition around (|δ|) ≈ 45°. Defining an effective gravitational force ∼1 – (Σ _B), the average magnetic-field-reduced star formation efficiency is at least a factor of two smaller than the free-fall efficiency. For about one fourth of the sources the average efficiency drops to zero. The force ratio Σ _B can further be linked to the normalized mass-to-flux ratio, yielding an estimate for the latter one without the need of field strength measurements. Across the sample, a transition from magnetically supercritical to subcritcal is observed with growing misalignment (|δ|)

[en] Massive stars (M > 8 M _☉) typically form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. We investigate the role of magnetic fields in this process through dust polarization at 870 μm obtained with the Submillimeter Array (SMA). The SMA observations reveal polarization at scales of ≲0.1 pc. The polarization pattern in these objects ranges from ordered hour-glass configurations to more chaotic distributions. By comparing the SMA data with the single dish data at parsec scales, we found that magnetic fields at dense core scales are either aligned within 40° of or perpendicular to the parsec-scale magnetic fields. This finding indicates that magnetic fields play an important role during the collapse and fragmentation of massive molecular clumps and the formation of dense cores. We further compare magnetic fields in dense cores with the major axis of molecular outflows. Despite a limited number of outflows, we found that the outflow axis appears to be randomly oriented with respect to the magnetic field in the core. This result suggests that at the scale of accretion disks (≲ 10³ AU), angular momentum and dynamic interactions possibly due to close binary or multiple systems dominate over magnetic fields. With this unprecedentedly large sample of massive clumps, we argue on a statistical basis that magnetic fields play an important role during the formation of dense cores at spatial scales of 0.01-0.1 pc in the context of massive star and cluster star formation.